(1) Field of the Invention
The present invention relates to a hold-response-type display device such as a liquid crystal display device or an organic EL display and, more particularly, to a display device suitable for a large screen and display of a moving picture.
(2) Description of the Related Art
In a liquid crystal display device, a number of pixels are formed in a matrix and are surrounded by data signal lines extending in the vertical direction and arranged in the lateral direction and scan lines extending in the lateral direction and arranged in the vertical direction. When a scan line is selected by a scan driver, image data of one line is written at once to pixels on the selected scan line by a data driver. As the size of the screen of a liquid crystal display for TV is increasing, the number of scan lines is also increasing. On the other hand, the period of one frame is unchanged. Therefore, when the screen becomes larger, the time of writing image data to pixels is short, and a phenomenon occurs such that reproduction of an image is insufficient.
As a countermeasure against insufficient time to write data to pixels, there is means for compensating the insufficient write time by pre-writing data to a scan line immediately preceding the scan line to be selected. The technique is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. H8-248385.
A liquid crystal display device and an organic EL display device have another problem such that the picture quality of a moving picture is not sufficiently high due to the following phenomenon.
From the viewpoint of display of moving pictures, the display devices are largely divided into an impulse-response-type display and a hold-response-type display. The impulse-response-type display is of a type in which luminance response decreases immediately after a scan like the light emission characteristic of a cathode ray tube. The hold-response display is of a type in which luminance based on display data is held to the next scan like a liquid crystal display.
Although the hold-response-type display can display a flicker-free still picture having excellent quality, in the case of displaying a moving picture, there is a problem of low picture quality. A so-called moving picture blurring that the surrounding of a moving object in a moving picture is blurred occurs. The moving picture blurring occurs due to a so-called afterimage. When the user moves his/her visual line along a moving object, the user interpolates display images before and after the display image whose luminance is held. Therefore, even if the response of the display is increased, the moving picture blurring is not completely prevented. To solve the problem, a method of updating a display image at a shorter frequency and a method of cancelling an afterimage by insertion of a black screen so that the display becomes like an impulse-response-type display are effective.
On the other hand, a representative display requested to display a moving picture is a television receiver set. The frequency of the television receiver set is a standardized signal such as 60 Hz in an NTSC signal and 50 Hz in a PAL signal. In the case where the frame frequency of a display image generated on the basis of the frequency is set to 60 Hz or 50 Hz, due to low frequency, the moving picture blurring occurs.
To reduce the moving picture blurring, as a technique of updating an image in cycles shorter than the above, there is a method of increasing the scan frequency and generating display data of an interpolation frame on the basis of display data between frames, thereby increasing the image updating speed. Japanese Patent Application Laid-Open Publication No. 2005-6275 discloses the technique. The technique of inserting a black frame (black image) includes a technique of inserting black display data between display data (hereinbelow, simply called black display data insertion method) and a technique of repeating turn-on and turn-off of a backlight (hereinbelow, simply called blink backlight method). For example, the technique is disclosed in Japanese Patent Application Laid-Open Publication No. 2003-280599.
When an attempt is made to shorten the write time and to reduce the moving picture blurring by applying the above-mentioned techniques, a new problem occurs. For example, the problem of the countermeasure against the moving picture blurring is as follows.
In the interpolation frame generating method, display data which is not originally present is formed. Accordingly, in an attempt to generate more accurate data, the circuit scale enlarges. On the other hand, when the circuit scale is suppressed, an interpolation error occurs.
In the method of inserting the black frame, in principle, no interpolation error occurs. Also from the viewpoint of the circuit scale, the method is more advantageous than the interpolation frame generating method. However, in both of the black display data insertion method and the blink backlight method, luminance at all of levels of gray decreases only by the amount of black frames.
As a method obtained by improving the method of inserting black frames, there is a method of inserting black data while suppressing decrease in luminance by forming one frame by two fields. In the improved method, specifically, two field memories are prepared. By writing image data of two fields to a liquid crystal display at a frequency twice as high as that of an input signal, one frame is formed by the two fields. FIG. 2 shows the relation between the tone and luminance in the two fields.
In FIG. 2, 256 gray scales are displayed. The first field undertakes the luminance when the gray scale is 171 or less. When the gray scale is 171 or less, an output from the second field may be zero. That is, when the gray scale is 171 or less, black data can be inserted without accompanying decrease in luminance. When the gray scale exceeds 171, for example, when the gray scale shown in FIG. 2 is 200, image data is output also from the second field. However, the luminance is lower than that of the first field, so that moving picture blurring can be lessened.
However, in the method, one frame is formed by two fields. Consequently, the time of writing image data to pixels is the half of the time of a normal driving method, so that increase in the unapplied voltage due to decrease in the data write time becomes a big issue. When the technique of Japanese Patent Application Laid-Open Publication No. H8-248385 is applied to the method of forming one frame by two fields of different luminance, a problem occurs such that the unapplied voltage rather increases in some cases.